This invention relates to the treatment of Young Tree Decline in citrus trees.
Young Tree Decline is a relatively recent and very serious disease of citrus trees, particularly the orange and grapefruit trees of the southeastern United States. Where it occurs in trees 10 or more years of age, particularly in sandy soils, it is more commonly termed "Sand Hill Decline." Other synonymous expressions for the affliction are "Rough Lemon Decline" and "Citrus Blight". As employed herein and in the appended claims, "Young Tree Decline" is intended to include these other terms.
Young Tree Decline has destroyed thousands of acres of sweet oranges and grapefruit in Florida and continues to spread rapidly. Symptoms include zinc-deficiency such as chlorosis patterns on leaves, dull green foliage, unseasonal wilt, small leaves in delayed growth flushes, progressive dieback of twigs and branches that may result in stunted trees with thinly-foliated live wood and masses of deadwood. The cause of this disease is not known.
Trees on rough lemon rootstock appear to be the most susceptible to the disease, even though trees on other commonly used rootstocks are not known to be immune to the disease. Since about 60 percent of the citrus trees in Florida are budded to rough lemon rootstock, the disease constitutes a very serious threat to the Florida citrus industry. Trees affected by Young Tree Decline may become commercially unproductive or die within a period of 4 to 24 months.
The owners of citrus groves affected by Young Tree Decline have been limited essentially to one of two choices: (1) remove and dispose of the affected trees, replacing them with healthy young trees, or (2) use the techniques of scion-rooting by girdling the affected trees a few inches above the bud union, applying a fungicide to the girdled area, piling soil around the tree to a point 6 or more inches above the girdle, keeping the soil mound intact against erosion by wind or water and also keeping the mount moist by irrigation during dry or rainless periods, to bring about growth of roots of the scion variety in the girdled area to supplement or replace the roots of the rootstock variety. Both of these choices have significant economic disadvantages.
Removal and disposal of affected trees often costs from $8 to $20 per tree, varying with tree size and distance the trees must be moved for burning or other disposal. Such removal must be followed by replacing the afflicted tree with a young healthy tree from the grower's own nursery or one bought from a commercial citrus nursery. Such young trees, scattered through the grove, must be watered frequently by tank truck during the first one or two years in the grove and hand fertilized separately from the bearing trees remaining in the grove. It may require five to ten years before the new trees will produce enough fruit to pay the cost of their purchase, planting and care up to bearing age. Where a great many trees are removed from the grove, there is a large drop in fruit production and consequently greatly reduced income from sales of fruit.
Scion-rooting has been tried by a number of growers with highly variable results. Many trees given this treatment die from various causes, such as failure to maintain the soil mound in place well above the girdle, failure to keep the soil mound moist enough for good root growth, or from the fungus disease "foot rot" developing on the tree trunk inside the mound. Many trees that survive this treatment fail to grow and/or produce fruit satisfactorily.